Gas turbine engines are known to include a compressor for compressing air; a combustor for producing a hot gas by burning fuel in the presence of the compressed air produced by the compressor, and a turbine for expanding the hot gas to extract shaft power. Diffusion flames burning at or near stoichiometric conditions with flame temperatures exceeding 3,000° F. dominate the combustion process in many older gas turbine engines. Such combustion will produce a high level of oxides of nitrogen (NOx). Current emissions regulations have greatly reduced the allowable levels of NOx emissions. Lean premixed combustion has been developed to reduce the peak flame temperatures and to correspondingly reduce the production of NOx in gas turbine engines. In a premixed combustion process, fuel and air are premixed in a premixing section of the combustor. The fuel-air mixture is then introduced into a combustion chamber where it is burned. U.S. Pat. No. 6,082,111 describes a gas turbine engine utilizing a can annular premix combustor design. Multiple fuel nozzles and associated premixers are positioned in a ring to provide a premixed fuel/air mixture to a combustion chamber. A pilot fuel nozzle is located at the center of the ring to provide a flow of pilot fuel to the combustion chamber.
The design of a gas turbine combustor is complicated by the necessity for the gas turbine engine to operate reliably with a low level of emissions at a variety of power levels. High power operation requires greater quantities of fuel making the lean pre-mix combustion principle, and therefore emissions requirements, significantly more difficult. Low power operation conversely challenges operational stability tending to increase the generation of carbon monoxide and unburned hydrocarbons due to incomplete combustion of the fuel. Under all operating conditions, it is important to ensure the stability of the flame to avoid unexpected flameout, damaging levels of acoustic vibration, and damaging flashback of the flame from the combustion chamber into the fuel premix section of the combustor. A relatively rich fuel/air mixture will improve the stability of the combustion process but will have an adverse affect on the level of emissions. A careful balance must be achieved among these various constraints in order to provide a reliable machine capable of satisfying very strict modern emissions regulations.
Dynamics concerns vary among the different types of combustor designs. Gas turbines having an annular combustion chamber include a plurality of burners disposed in one or more concentric rings for providing fuel into a single toroidal annulus. U.S. Pat. No. 5,400,587 describes one such annular combustion chamber design. Annular combustion chamber dynamics are generally dominated by circumferential pressure pulsation modes between the plurality of burners. In contrast, gas turbines having can annular combustion chambers include a plurality of individual can combustors, such as the combustor described in the aforementioned '111 patent, wherein the combustion process in each can is relatively isolated from interaction with the combustion process of adjacent cans. Can annular combustion chamber dynamics are generally dominated by axial pressure pulsation modes within the individual cans.
Staging is the delivery of fuel to the combustion chamber through at least two separately controllable fuel supply systems or stages including separate fuel nozzles or sets of fuel nozzles. It is known in a can annular combustor of the type described in the aforementioned '111 patent to provide fuel to the ring of main fuel burners through two different stages, alternating the stages between adjacent burners around the ring. In this manner, a degree of control is afforded to the operator to affect the combustion conditions by independently varying the amount of fuel supplied to each stage as the power level of the engine is changed. The burners are symmetrically staged around the longitudinal axis of the combustor so that the flame produced by both stages is the same. Improved performance is achieved by increasing the power level of the combustor primarily with one main fuel stage as the second main fuel stage is kept at a reduce fuel flow rate. Once the first stage is at full power, the second main fuel stage is ramped up to full power. The burners of both stages are identical, so the flame conditions in the combustor are the same regardless of which stage is the first stage to be ramped upward.
The demand to decrease exhaust emissions continues, thus it is desired to operate a gas turbine engine with little or no diffusion flame. The control of combustion in a gas turbine engine becomes very challenging without the stabilizing effects of a pilot diffusion flame. Improved techniques for controlling the combustion conditions of a gas turbine engine are needed.